have 2 WD Red 4TB drives, what is the best enclosure to use these with my Mac Mini? does it matter if its USB 2/3 as I don't think the drives are fast enough?
USB 2.0 offers a maximum of 40 MB/s read or write bandwidth, the 4 TB RED drives offer at least twice of that with their 110 MB/s read and write speeds, thus USB 3.0 is the better solution.
I do not have any recommendation for two drive bays, as I have all my drives internally and in one (soon there will be a second) quad drive bay from Fantec, but I also have around 20 HDDs.
That HDD bay from Fantec supports 4 TB HDDs and if you ever want another HDD or even two, you can easily add them to that case. It is also quite quiet at the lowest fan speed setting, quieter than my Mac with its custom hardware.